Don't throw out charcoal. Throw it in.

When woody matter is burnt in partial oxygen, it turns into charcoal. When charcoal is incorporated into the soil, it is found to improve its quality in many ways. Vast stretches of this black soil called ‘Terra Preta’ (dark earth) was discovered in the Amazon by soil scientists in the 1950s. Tera Pretta is considered to be one of the most productive and fertile soils ever found. Charcoal, on its own, does not add any nutrition to the soil. But when it is added to nutrient-rich soils, it helps preserve the nutrients and ration them out to the plants over a long period of time, i.e. for decades rather than a couple of seasons. Let’s see how Terra Preta works!

It slows down the mineralization of organic matter

When organic matter gets composted (breaks down) into nutrients, we call it ‘mineralisation of organic matter’. In hot and humid temperatures like the Amazon and India, mineralization happens more rapidly than the plants are able to take in the nutrients. It is like someone preparing lots more food than can be consumed by the members of a household. Charcoal slows this process of mineralization down in an interesting way.

Coarse lumps of charcoal are full of crevices and holes. 1 gram of charcoal is found to have upto 500 square metres of internal surface area! This triggers the growth of a lot of good soil bacteria, fungi, mycorrhiza etc. Though the overall populations of fungi and bacteria are high in the soils, the presence of abundant carbon makes the microorganisms live and reproduce at a slowed pace. This slows down the turnover rate of organic matter in the soil, so composts last much longer.

It prevents the leaching of nutrients

When a soil rich in nutrients is continually exposed to incessant and heavy rains, like in the Amazon and India (during the monsoons), the unused nutrients get washed away leaving the soil infertile.

The carbon compounds in charcoal form loose chemical bonds with soluble plant nutrients so they are not as readily washed away by rain and irrigation. It absorbs excess water, nitrogen and other nutrients and releases them slowly as and when the plants need it.

It traps and stores heavy metals

The crevices trap and hold heavy metals present in contaminated soils.

It neutralises acidic soil.

Charcoal is alkaline and hence helps make soils less acidic and more ready to support plant life. It is good for reducing the acidity of acid soils. It helps neutralize the impact of acid rains on agricultural soils.

Abundant charcoal in Chennai.

Charcoal waste from the iron walla
piled up on the pavement of my street.

The local iron-wallas located in almost every street of Chennai throw away bagfuls of charcoal into the dustbins as waste everyday. I’ve started collecting every bit of it from my street fellow. He’s very happy to store it over a week’s time and give it away to me.

You can also burn woody matter like coconut shells, coconut trunk, etc. to generate charcoal. Here is a simple way. Make a heap of the woody material and set fire to it. Allow it to burn until the high flames come down. Then put out the fire by pouring plenty of water (or sand / soil) on it.

But be careful not to burn away too much organic matter, for we need them to feed the microorganisms in the soil too by composting it.

How to use charcoal in urban farming?

A mixture of coco-peat, compost
and powdered charcoal makes
good potting mixture 

This usually comes mixed with ash. Seive it and separate the ash from the charcoal pieces. Sprinkle the ash on the soil only occasionally, since you don’t want to end up with a soil that is too alkaline. Keep adding a handful of charcoal (pounded into 1 cm cubes) to your compost pile periodically. They soak up excess water, encourage the growth of beneficial microorganisms and remove bad odour resulting from any anerobic activity. You can add this compost (already mixed with charcoal pieces) to your soil. Try to keep the soil : charcoal ratio as roughly 10 : 1

My little helper pounding charcoal

If you are directly applying charcoal into the soil, you need to charge it before doing so. You can soak it in urine (human or animal), EM solution, panchakavya, amrit pani, vermiwash, compost tea, etc. This will help the charcoal soak up the nutrients and release them to the roots slowly. I’ve been collecting and using human urine. There is absolutely no smell since the nitrogen in the urine immediately gets absorbed into the charcoal pores. And what a great way to use what goes as waste into our sewers!

Some useful links

Terra Preta http://en.wikipedia.org/wiki/Terra_preta

Secret of Eldorado is a documentary about the discovery of ‘Terra Preta’ in the Amazon http://www.youtube.com/watch?v=0Os-ujelkgw

Dr. Sai Bhaskar Reddy shares his terra preta experiments and their results in his blogs http://e-terrapretarooftopexp.blogspot.in/ and http://e-alkalinesoilsterrapreta.blogspot.in/

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